Individuals having a sibling with Non-insulin dependent Diabetes Mellitus, and identified as having IGT, have a 50 percent chance of becoming diabetic within 3 years. Patients identified at such high risk will be randomly assigned to placebo or a low dose of Glipizide (1.25-2.5 mg/day) to determine if this prophylactic treatment with sulfonylurea will prevent or delay the development of NIDDM.